Three to six people, plus one facilitator who doesn't take part.
Smaller than three and you miss the friction that triggers good ideas. Larger than six and you lose people: some never speak, others dominate, and the conversation fragments.
Pick people from different layers and disciplines. Not necessarily the supposedly most creative ones. Often, people from operations or customer service give the sharpest input, because they hear what's actually going on in the business.
One exception: if you really want to involve everyone in a bigger team, run two parallel sessions of six and compare the output afterwards.